1. Field of the Invention
The present invention relates to a corona discharger, more particularly, to a corona discharger which can be used as a corona discharger, a transfer charger, or a charge eraser, for use in an electrophotographic copying machine.
2. Description of Related Art
Corona dischargers have been used for use in, for example, an electrophotographic copying machine, as a charger for uniformly electrifying a photoconductive drum, a transfer charger for transferring a visible toner image formed on the photoconductive drum onto a piece of copying paper, or a charge eraser for removing the charge on the photoconductive drum.
A conventional corona discharger comprises a discharge wire to which a high voltage is applied, and a shielding plate for stabilizing the corona discharge produced around the discharge wire. For example, when the corona discharger of this type is used as a corona charger, the discharge wire is provided apart from the surface of the photoconductive drum by a predetermined constant distance so that the axis thereof becomes parallel thereto. If a high voltage is applied between the discharge wire and the photoconductive drum in this state, the corona discharge is started in the vicinity of the discharge wire, and a current due to the produced ions flows between the discharge wire and the surface of the photoconductive drum. Then, the charge is stored on the surface of the photoconductive drum.
Accompanying with the corona discharge, the ozone is produced. Since the ozone has a property of deteriorating the photoconductive drum by acting the surface thereof, it is necessary to prevent the produced ozone from harmfully influencing the photoconductive drum.
In a corona discharger proposed in the U.S. Pat. No. 3,777,158, a distance between a discharge wire and a shielding plate is set so as to be smaller than a distance between the discharge wire and a photoconductive drum, and then, more larger corona current flows onto the shielding plate than that flowing onto the photoconductive drum. As a result, an air stream induced by the produced ion stream flows toward the shielding plate, and then, the ozone produced by the corona discharge is discharged through an outlet formed in a shielding plate to the side opposite of the discharge wire to the photoconductive drum.
However, in the corona discharger proposed in the above U.S. Patent, since almost all the current caused by the corona discharge flows onto the shielding plate, much power is consumed uselessly and uneconomically. In order to obtain a high charging ability, it is necessary to provide a higher voltage source having a higher output power, and the cost of manufacturing the corona charger increases. In addition to this, since the whole discharge amount of the corona discharge becomes relatively large, much ozone is produced. Therefore, even though the produced ozone is efficiently discharged, harmful influence to human body can not be neglected.
In order to prevent a large current from flowing onto the shielding plate, there has been proposed a corona discharge constituted so that almost all the current flows between a discharge wire and a photoconductive drum by applying to the shielding plate a bias voltage having the same polarity as that of a voltage to be applied to the discharge wire. In this case, the voltage to be applied to the discharge wire is relatively low. However, since the ion stream caused during the corona discharge flows from the discharge wire toward the photoconductive drum, an air stream induced by the produced ion stream flows from the vicinity of the discharge wire toward the surface of the photoconductive drum. Then, almost all the ozone produced by the corona discharge is transported toward the surface of the photoconductive drum by the aforementioned air stream, resulting in deterioration of the photoconductive drum.
Furthermore, there has been a corona discharger comprising an electrically insulating plate on the inner surface of the shielding plate so as not to flow the ion current onto the shielding plate. However, the corona discharger of this type has the following problems. In the corona discharger, the insulating plate is electrified, and an alien substance and a dust adhere to the insulating plate while the corona discharge is stopped, resulting in ununiform electrifying during the corona discharge. Further, since the electric field induced around the discharge wire varies depending on the electrifying amount of the insulating plate, the corona discharger has such a disadvantage that the corona discharge becomes unstable.
Furthermore, there has been proposed in the Japanese patent laid open publication (JP-A) No. 64-82059 published on Mar. 28, 1989, a charger wherein there is set a relatively long distance between a side plate of a shielding member and a discharge wire in order to decrease the current flowing from the discharge wire to the side plate of the shielding member. The charger is capable of decreasing useless corona discharge, however, there is such a problem that the charger is scaled up since it is necessary to provide the side plate of the shielding member so as to be apart from the discharge wire by a predetermined distance.